GRADING AND SCREENING 231 the division points the inventors modified the elementary inclined plane shaking deck. The plane surface was replaced by a corrugated one, the corrugations being parallel to a more or less degree to the line of action of the head motion. The first effect of such corrugations is to produce interstitial settlement the fines going to a point nearest the deck while the coarser particles remain at points nearer the top of the mass resting on the corrugations. By this action also the fines being more nearly in contact with the deck partake more nearly of its motion and proceed forward more rapidly than the coarse material does. The latter kind being less under the influence of the advancing motion proceeds more down the deck than along it and there is consequently better spreading and more capacity. On the types of machine for fine grading, the larger grains in an individual corrugation, at the moment of rolling, may be considered to be resting on an inclined plane of fine material and just as on a simple plane surface they roll down faster than the finer ones do. On the machines for coarse the effect of the corrugations depends largely on whether the vertical resultant line of gravity falls inside or outside of the edge of the corrugation. If outside the particle will leave the corrugation on which it is resting for the one below. This effect is very noticeable on the coal machine. A glance at Fig. 8 will make this action clear. In the coarse-grading machines the corrugations have FIG. 8.—Irregular body a slight upward angle corresponding with the upward thrust on sizer. given by the mechanism which shakes the deck. At certain intervals, depending upon the number of sizes desired, the general plane of the deck formed by the edges of the corrugations is broken into a plurality of such planes which are inclined to one another. Each set of corrugations yields a grading. The staggered arrangement of the corrugations will be plain by referring to Fig. 6. In addition the successive sets of corrugations dimmish in depth and pitch. The effect of inclining the corrugations by raising the forward ends of each set and depressing the rearward ones below the general plane of the deck, is to cause the coarser pieces to roll back, while the fines continue to advance and makes for quick separation and capacity. The effect of narrowing the corrugations is to crowd out the coarse material while the fines continue to advance and also makes for capacity. The same effect would be obtained if the corrugations were continuous and with regular inclination from end to end, but with gradually diminishing depth and pitch but such a construction would be inferior in quick separation and capacity to the broken surface arrangement. It would have, the advantage that the division into sizes would not be arbitrary and could be changed at will. On the corrugated belt machine for very fine material the advantages secured by diminishing the depth and pitch of corrugation cannot be obtained and the action of this type of machine is inferior to the others on this account. On the score of destructive vibration the bolt machine is superior to the other typos. On the shaking types the vibration is nob only destructive to the machine but probably affects the grading work when the greatest precision obtainable is desired. In the writer's opinion balancing would overcome this defect which is a serious one but nothing has been done to redesign the shaking types with this feature in mind. The machine has not attained a more prominent position principally owing to the reason that it has not to dato duplicated the work of screens when precise work is required. This will be evident from the following screen tests. Test on Carborundum with type-2 machine (not illustrated). Rate of feed, 15 tons in 24 hr.